1. Field of the Invention
This invention relates to a method of producing metallic materials for components of nuclear reactors subjected to neutron irradiation, for example, metallic materials used as reactor vessels for fast breeder reactor, light water reactor, nuclear fusion reactor and so on. More particularly, it relates to a method of producing metallic materials containing a slight amount of boron (B), such as carbon steel, low chromium.molybdenum (Cr--Mo) steel, ferritic high chromium steel, stainless steel, superalloys and the like.
2. Related Art Statement
As is well-known, ASTM A533 B Cl. 1 steel, A508 Cl. 3 steel and the like are, for example, used as a low carbon steel for reactor pressure vessel. On the other hand, low Cr--Mo steels, ferritic high chromium steels and ferritic stainless steels are considered to be applied to nuclear reactors, particularly fast breeder reactor and nuclear fusion reactor because they are cheap in the cost and excellent in the properties as compared with austenitic stainless steels. Furthermore, superalloys such as Inconel, Incolloy and the like have excellent thermal resistance and oxidation resistance, so that they are considered to be applied to nuclear reactors, particularly nuclear fusion reactor.
Moreover, the austenitic stainless steels have excellent high-temperature strength and corrosion resistance, so that they are used for various structural components in the nuclear reactor since early times. Particularly, their use for reactor vessels in fast breeder reactor and light water reactor subjected to thermal neutron irradiation will be expected.
In the austenitic stainless steel, it is known from, for example, PNC Technical Review No. 50 or Japanese Patent laid open No. 53-88,499 that the addition of B makes carbides fine and stable to restrain intergranular precipitation of carbide and strengthen grain boundaries, resulting in the improvement of strength and ductility as well as workability.
As described above, the addition of B to the austenitic stainless steel is effective from viewpoints of the strengthening of grain boundaries and the like, but has the following problems.
In general, boron is composed to two isotopes .sup.10 B and .sup.11 B as natural boron, a natural content of which is about 19.6% .sup.10 B and about 80.4% .sup.11 B. Among these isotopes, .sup.10 B is particularly large in the thermal neutron absorption, so that when the B-containing austenitic stainless steel is used in a vessel of a nuclear reactor subjected to thermal neutron irradiation, .sup.10 B is transmuted by .sup.10 B (n, .alpha.).sup.7 Li nuclear reaction even under thermal neutron irradiation at a relatively small dose of about 10.sup.17 n/cm.sup.2 into helium (He). This He promotes the occurrence and propagation of creep cracking, resulting in the creep embrittlement.
Furthermore, austenitic stainless steels containing intentionally no boron contain B of at least about few ppm through the usual steel-making process. In this case, there is a fear of causing creep embrittlement due to .sup.10 B transmutation under thermal neutron irradiation as mentioned above.
In the metallic materials other than the austenitic stainless steel, such as carbon steel, low Cr--Mo steel, ferritic high chromium steel, ferritic stainless steel or superalloy, B is added for improving the creep rupture strength or hardenability. Even if it is intended not to intentionally add B, the above metallic material usually contains at least about few ppm of B as an impurity through the refining process. In any case, when these metallic materials are applied to the reactor vessel used under thermal neutron irradiation, .sup.10 B is transmuted by .sup.10 B(n, .alpha.).sup.7 Li nuclear reaction to He gas likewise the case of the austenitic stainless steel. Such He atoms not only cause the creep embrittlement as mentioned above, but also result in the degradation of mechanical properties such as reductions of high-temperature ductility and creep rupture strength and the like.
Therefore, it is demanded to develop metallic materials for the components of nuclear reactors, which contain a boron ingredient having a .sup.11 B content larger than that of natural boron and may avoid the formation of He due to the .sup.10 B transmutation. In this connection, .sup.11 B-enriched compounds, which are suitable as a starting boron ingredient required for the industrial production of the above metallic materials used in the nuclear reactors, have not hitherto been produced.
As a method of producing .sup.11 B-enriched compound as compared with the natural boron, there are a method wherein boric acid is precipitated from sodium borate, calcium borate, crude boron or the like as a boron starting material, purified through recrystallization and separated by utilizing the weight difference between .sup.10 B and .sup.11 B, a method wherein a boron halide is formed from boric acid and separated by utilizing the difference in chemical characteristics thereof, and the like. In the latter method, the .sup.11 B-enriched halide is converted into metallic boron by a direct metal reduction process, a molten salt electrolytic process or a hydrogen reduction process.
In case of adding .sup.11 B to the metallic material, there is an adopted method wherein boric acid or metallic boron is directly added to the metallic material in the melting and refining thereof. However, boric acid is liable to surface upward in molten steel because of the poor wettability and the small specific gravity, so that it is very difficult to add boric acid at an adequate amount and hence the yield of .sup.11 B is low. On the other hand, the addition of metallic boron has some weak points that it is liable to surface upward because of the small specific gravity and is high in the production cost because of the complicated production process, resulting in the economical demerit in using this metallic material for the components of nuclear reactors.